Complete explanation to Dynoing

[sracing]

Originally Posted by UncleBob

If you hit redline at 140mph, and redline is 14K RPM's, then its a rather simple formula to figure out torque based off the HP curve

Yes if you know any RPM at any Speed then you have all you need in that particular gear.

Not sure how you can remove gearing and wheel diameter with no info, intertia or eddy. Without some reference to the RPM, torque is not measurable for the *engine*

Yes, you need drive ratios.

Jim

[sracing]

Originally Posted by Alastor187

I don’t see why it matters which one is measured first since both torque and power are useful measures.

It doesn't really as long as you understand the concepts and physics involved.

How is measuring strain first and then converting it to torque any different than measuring torque and then converting it into power?

See above. It doesn't really matter.

Technically speaking neither torque or power are really measured directly. More to the point, is strain even measured directly or is it voltage that is measured?

Well NOTHING is measured directly. I need the concepts of time, (RPM or controller clock), even with my "fish scale" dyno, I needed to convert the spring resistance to a dial gauge. Yes, the strain gauge resistance is simply measured with a calibrated voltage.

The real differences in the is HOW it is measured (inertia or brake) and that does make a difference.

For example, using two engines producing the same brake HP may be wildly different when measured in inertia mode. All other things equal, if one engine/drivetrain has much greater rotating mass, it may measure X HP in the brake mode while the lighter engine measures the same thing. However in inertia mode, the lighter engine will put more HP to the rear wheels.

This is critical in brake dyno readings. To compare one engine to another you have to use a similar acceleration rate (300 rpm per second as an example. This is often used as a standard).

There are a lot of different procedures when brake dynoing to either evaluate the inertia losses, or to negate them, depending upon what the planned use of the engine is. A diesel for example does pretty poorly in inertia mode, but does pretty well in steady state brake mode.

Jim

[UncleBob]

Originally Posted by Alastor187

I don’t see why it matters which one is measured first since both torque and power are useful measures.

I think this is the "what is better, HP or torque" argument contaminating the dyno theory. Some believe that Torque is everything, and they allow their understanding of dyno's to be corrupted by this....fanatical belief

Torque is a static measurement that has little bearing on real world measurments. That is partly why you need more info to calculate engine torque.

No it doesn't really matter which comes first, if you have all the data, you know both, and having both is good. But some like to argue about which comes first as if it matters, and they don't like the reality. HP comes first

[KiwiBacon]

Originally Posted by UncleBob

common misconception.

It measure rear wheel torque, which is then calculated into HP, and then with an RPM reference, is calculated into torque.

Its not a simple "this happens first" though, since HP and torque are forever intertwined. The bigger issue is calculating out the gearing.....its much simpler to do that by caluclating the HP first.

For example, if you have no tach lead, you can't calculate a torque reading for the engine. But you can print a HP readout. That should be pretty telling on what happens when

I disagree on that.
Most vehicles are dyno'd in a straight through gear (1:1) to minimise losses through the gearbox.
So the only things necessary to convert from wheel force to flywheel torque is the diff ratio and tyre size. Both are easy to find without needing a rpm feed from the engine.

The torque and HP curves at the wheels will give exactly the same shape as the torque and HP curves at the flywheel. They just need scaled, which depends on the gearing.

[KiwiBacon]

Originally Posted by UncleBob

explain why this dyno chart is impossible.....no tach lead whatsoever

Easy.
Power = Force * Velocity.
can also be written as
Power = Torque * rotating speed

The torque at the rollers is measured. The speed they are rotating at is directly measured also.
From this power is calculated.

[Alastor187]

Originally Posted by sracing

The real differences in the is HOW it is measured (inertia or brake) and that does make a difference.

For example, using two engines producing the same brake HP may be wildly different when measured in inertia mode. All other things equal, if one engine/drivetrain has much greater rotating mass, it may measure X HP in the brake mode while the lighter engine measures the same thing. However in inertia mode, the lighter engine will put more HP to the rear wheels.

This is critical in brake dyno readings. To compare one engine to another you have to use a similar acceleration rate (300 rpm per second as an example. This is often used as a standard).

There are a lot of different procedures when brake dynoing to either evaluate the inertia losses, or to negate them, depending upon what the planned use of the engine is. A diesel for example does pretty poorly in inertia mode, but does pretty well in steady state brake mode.

Jim

That is an interesting point.

[UncleBob]

Originally Posted by KiwiBacon

I disagree on that.
Most vehicles are dyno'd in a straight through gear (1:1) to minimise losses through the gearbox.
So the only things necessary to convert from wheel force to flywheel torque is the diff ratio and tyre size. Both are easy to find without needing a rpm feed from the engine.

The torque and HP curves at the wheels will give exactly the same shape as the torque and HP curves at the flywheel. They just need scaled, which depends on the gearing.

I didn't say its impossible, I said it requires more info. If you don't have the exact wheel circumference, and the exact final (total) gear ratio, you won't have the exact RPM, and without the exact RPM, your torque numbers will be wrong.

You will have a curve, but without accurate numbers, its not terribly helpful

[UncleBob]

Originally Posted by KiwiBacon

Easy.
Power = Force * Velocity.
can also be written as
Power = Torque * rotating speed

The torque at the rollers is measured. The speed they are rotating at is directly measured also.
From this power is calculated.

exactly what I was refering to. The person I was responding to said it wasn't possible

[stick99]

I do not believe it is possible to find HP without knowing a RPM. It could be the RPM of the Engine, or of the dyno, as long as you have the Calculations right. KevintheNerd did a decent explanation on how RPM and torque change though gearing here:

http://www.saturnfans.com/forums/sho...d.php?t=103238

For Engine dynos, the engine is connected directly to either a water or electric (a.k.a. eddie current) brake. This brake puts a load (torque) on the engine. Because of Newton's third law, this load against the puts a equal and opposite torque on the housing of the brake. The torque on the brake is measured often by a strain gage at a known distance from the break. ex: 100lbs @ 2 ft from the center of the rotating shaft equals 200 ft*lbs of torque. This torque that you just found is the torque that the engine is applying to the brake.

For chassis dynos, there are a few ways to do this: one, take the drums that your wheels turn and hook up a break to them, but this complicates things, so probably not used. Two, the drums' RPM can be constantly measured. Using numerical analysis of the drum's rpm over time, one can calculate the angular acceleration of the drums. Using:

Torque = m*r^2*a/2

where "m" is mass of the drums, "r" is the radius of the drums, and "a" is the angular acceleration. Now you know the torque applied to the drums


If the engine is connected without any gearing to the dyno, the torque found above is the engine output torque. If there is gearing between the engine and dyno, then by using the gear ratios, one can calculate the engine output torque. If you recorded what RPM produced what amount of torque, then you can create a Torque vs RPM graph.

As for the HP, using the torque vs rpm data, you can calculate HP using this equation:

Power (hp) = Torque (ft*lb) * Rotational Speed (rpm) / 5252

That equation is from kevinthenerd, see the above link for an explanation of how to get that formula.

I hope this helps, and i believe it is accurate.

[UncleBob]

thats rear wheel torque, not engine torque. There's a large difference between the two

[KiwiBacon]

Originally Posted by UncleBob

thats rear wheel torque, not engine torque. There's a large difference between the two

The difference between the two is the drive ratio.

[stick99]

you can calculate engine torque from rear wheel torque, right? given you know all the correct ratios.

input RPM = x*output RPM

where "x" is the gear ratio.

input torque = output torque/x

this is true because torque and rpm are inversely proportional to each other when dealing with gears.

it won't be exact because you have friction loses and will be extremely difficult to account for the angular acceleration of all the rotating mass in the car. But if the drum that the wheels turn have a large amount of mass compared to the the rotating parts in the car, those losses will not throw your data off too much.

[UncleBob]

yes, if you have the added info. As I already said, you need the added info, or a tach lead, before you can calculate the engine torque

Engine HP doesn't require it